The present invention relates to a superconducting device which operates at a cryogenic temperature, and more particularly to a superconducting switching device in which the numbers of superconducting and normal conducting electrons which tunnel through a semiconductor are controlled by a voltage applied to a control electrode.
As a superconducting device in which a semiconductor and a superconductor are combined, there has been known a hydrid Josephson field effect transistor (JOFET) proposed by T. D. Clark et al, (J. Appl. Phys. 51(5) 2736-2743 (1980)). The JOFET has a structure in which the semiconductor (normal conductor) and the superconductor are joined. Electron pairs or Cooper pairs in the superconductor leak into the semiconductor by a depth corresponding to a coherence length. It is the principle of the JOFET that the Cooper pairs having leaked into the semiconductor are moved in the direction within the plane of the semiconductor. Since, in this JOFET, the Cooper pairs are supplied from a gate electrode, the current gain cannot exceed 1 (one). Therefore, the circuit gain is disadvantageously small.
As other superconducting devices in which a semiconductor and a superconductor are combined, there have been ones disclosed in Japanese patent application Laying-open No. 57-176781 and No. 57-106186. The device of the former uses a semiconductor material which can fall into a superconducting state at a temperature below a superconducting transition temperature T.sub.c. T.sub.c in this case is, in general, nearly equal to or lower than the temperature of liquid He (4.2 K). In order to stably operate a circuit, therefore, the device needs to be cooled below 4.2 K. The device of the latter has a structure in which a control electrode is disposed in the narrow space between superconducting electrodes, so that the fabrication of the control electrode is difficult.